This invention relates to a process for making porous polymeric products and to the products so-produced. More particularly, this invention relates to a process for making porous polymeric products from a solid nonporous polymeric structure formed of a homogeneous composition to produce a porous product that can include or be devoid of a solid nonporous layer.
Prior to the present invention, membranes have been produced by thermal or solution phase inversion, stretching, sintering or track etching. By far, the most common process for making microporous membranes is solution phase inversion (wet and/or dry) by which many commercially available membranes are made from polymers such as polyvinylidene fluoride (PVDF), cellulosics, polysulfones or polyamides (Nylons). Phase inversion membranes are made by dissolving a polymer in an appropriate solvent to form a solution. The solution is cast into a thin film, usually 25 um to 250 um thick, on a substrate. The film then is exposed to a suitable non-solvent in the gaseous (dry) or liquid (wet) phase. This latter step induces phase separation, i.e., precipitation of the polymer as a solid mass from the solvent phase. Phase separation conditions can be controlled to effect formation of a porous polymeric structure having a relatively uniform pore size distribution.
It has been proposed in U.S. Pat. No. 2,783,984 to form a polyamide microporous membrane by first forming a solution of polyamide in a mixture of a non-solvent and a solvent for polyamide at the point of incipient polyamide precipitation. An additive such as the non-solvent such as boric acid or citric acid is added to effect the desired precipitation and to form a self-sustaining non-transparent microporous membrane. In the process, as the first step, a homogeneous polyamide solution is formed. The solution is cast on a flat surface such as glass and heated to effect solvent evaporation and to form the microporous membrane. A similar process for forming polyamide microporous membranes is disclosed in U.S. Pat. No. 3,408,315.
U.S. Pat. No. 3,876,738 discloses a process for forming polymeric microporous embranes such as nylon membranes. A dope solution of the polymer is first formed and is directly cast onto a surface which can be positioned below the surface of a non-solvent for the polymer. The dope solution is quenched within the nonsolvent to effect polymer precipitation and membrane formation. Process control parameters are controlled to eliminate the prior art step of gelling in a high humidity atmosphere. The formation of a dope solution is required as the first step in the process.
U.S. Pat. No. 4,340,479 discloses a process for producing a skinless hydrophilic alcohol-insoluble polyamide membrane. In a first step, a solution of the polyamide membrane is formed and free of suspended particles. Nucleation of the solution is effected by the controlled addition to the solution of a non-solvent for the polyamide. The polyamide membrane produced is characterized by reversion from a hydrophilic material to a hydrophobic material which is not wet by water when heated to a temperature just below its softening point.
U.S. Pat. Nos. 4,203,847 and 4,203,848 also disclose processes for forming polymeric microporous membranes. In this process, a solution of the polymer is first formed. The solution then is passed into a bath of non-solvent for the polymer to effect polymer precipitation and membrane formation. The ratio of solvent to non-solvent in the bath is monitored and its composition is adjusted to a desired range.
It has been proposed in U.S. Pat. No. 3,839,516 to contact a crystalline polymeric film with a swelling agent and then stretching the film while the swelling agent is removed to form a porous film. The pores are unidirectional in shape due to the tensile force exerted on the film during stretching. Similarly, it has been proposed in U.S. Pat. No. 3,426,754 to subject a crystalline polymeric film to cold drawing in an amount of 10% to 300% of its original length and then heat setting the film under tension to form a porous film. The pores formed in this film also are unidirectional in shape due to the tensile force exerted on the film during cold drawing and heat setting.
It has also been proposed to treat a film formed from a nonhomogeneous polymeric composition which contains either an extractable composition or a releasable composition such as by solvent extraction or heating so that pores are formed in the polymeric film after the composition has been either extracted and/or released. The polymeric compositions which can be treated in this manner are limited since they require the presence of either the extractable composition or the releasable composition.
The porous membranes produced can be utilized in filtration in general or in specific utilizations such as for binding nucleic acids such as DNA, RNA, proteins, cells, or the like. Polyamide membranes are particularly useful for binding nucleic acids since they provide a large surface area and covalently bind the nucleic acid. In certain applications such as in direct blotting electrophoresis (DBE), a rigid substrate is desired for use as both conveyor and as the immobilizing matrix. Presently available polyamide membranes require that they be laminated to a rigid substrate. When wet, these laminates swell, wrinkle and delaminate.
Many of the processes described in the above-identified patents rely upon the formation of an initial casting solution which is then processed. The casting solutions are formed by admixing polymeric particles until the particles completely dissolved. Since the particles vary in size, the time required to solvate the individual particles will vary from batch to batch. The composition of these solutions must be controlled carefully in order to attain the desired polymer precipitation and membrane formation in the subsequent step of contacting the solution with a non-solvent for the polymer. This procedure requires that the process be conducted as a batch process wherein the process is initiated by forming the polymer solution. In addition, it is difficult to precisely control the product consistency from batch to batch since it is difficult to exactly reproduce the solution composition from batch to batch.
Therefore, it would be desirable to provide a process for forming polymeric porous products which is a continuous process rather than a batch process in order to improve efficiency. In addition, it would be desirable to provide such a process wherein the time that the solvent interacts with the solid polymer can be controlled precisely. This control would permit the formation of porous products having a more uniform surface porosity. In addition, it would be desirable to provide such a process wherein the average pore size can be controlled precisely and wherein the distance that the pores extend from the product surface into the product thickness can be controlled precisely. In addition, it would be desirable to provide a process which does not rely on the presence of extractable and or releasable compositions to form a porous structure. Furthermore it would be desirable to provide such a process which does not require the use of tensile forces such as in a stretching step to form a porous structure.